One of the methods for expressing a sense of volume or distance is blurring. For example, when a photograph is taken, an object just in focus is taken clear and degree of blurring is greater as the object is more separated from the focus. It is known that this blurring provides a photograph with a sense of distance.
One of the methods for expressing blurring by means of computer is dispersed beam tracing method. Another method to express blurring more simply is a method disclosed in the patent specifications of Toku-Kai-Hei 6-36025 and Toku-Kai-Hei 2-190898 in which the diameter of a blurring circle around each pixel is calculated based on the information of distance for the pixel and, thereby, spreading area of blurring extended to neighboring pixels is determined to show the sense of distance. Calculation of blurring process is carried out with dots of 3 by 3 in Toku-Kai-Hei 2-190898 and with dots of 5 by 5 in Toku-Kai-Hei 6-36025.
Of the methods referred to above, the dispersed beam tracing method requires a lot of time for calculation, such as for calculating on a multitude of beams to build up a blurred image related to one pixel. The method of calculating the effect of neighboring pixels on the pixel of interest can be carried out in shorter period than dispersed beam tracing method but it is associated with other problems.
For example, in the method disclosed in Toku-Kai-Hei 6-36025, the effect of neighboring pixels is taken into account with the use of a kernel of 5 by 5 to accomplish sense of distance, but pixels that can be considered with respect to the latter are those only two pixels remote from the pixel of interest. For calculation of data required to express a pixel, data on a kernel of 25 pixels including neighboring pixels have to be added after processing, resulting in heavy burden of calculation. 25 weight coefficients involved in the kernel have to be calculated for each pixel, thereby the burden of calculation increases. Weight coefficients for dots other than that in the center of the kernel are calculated by the following equation: EQU fw.sub.i,j =.vertline.(1-fw.sub.33)/(5.times.5-1).vertline..times.df.sub.i,j
The equation above can be transformed as follows: EQU fw.sub.i,j =(df.sub.33.times.df.sub.i,j)/(5.times.5-1)
This equation indicates multiplication of distance by distance. Therefore, repeating of filter processing seems to be needed, with the times of filtration set to control the degree of blurring. This repeated processing also increases burden of calculation.
In the method of Toku-Kai-Hei 2-190898, effect of surrounding pixels is taken into account based on a mask register of 3.times.3, but it is the effect of only pixels directly adjacent to the pixel of interest that can be dealt with. Moreover, addition of data on mask registers of 9 pixels including surrounding pixels subsequent to the processing is required to accomplish the calculation of data for expressing a pixel of interest. In this method, it is rather easy to select a suitable mask register based on the distance information but it is difficult to design the contents of mask register (weight coefficients) in accordance with the distance.
For a very far object, the effect of pixels remote as much as 10 pixels from the pixel of interest may contribute in view of characteristics of human vision. To encounter such an effect, the mask register has to be extended to 21.times.21, thereby addition of data on a mask register of 441 pixels consisting of surrounding pixels subsequent to the processing is required to calculate the data for expressing blurring of a pixel of interest, resulting in an unreasonable burden of calculation. A remarkable difficulty is expected to arise also in designing the contents of masks register in accordance with distance. Thus, real expression of sense of distance is limited in practical application of the method.